Diffraction is a common phenomenon experienced by any form of energy that propagates in a wave, but it’s most commonly associated with light. Diffraction of light takes place when light bends to pass through a narrow opening or around an obstacle. The resultant bending demonstrates the wavelike properties of light while also creating a distinctive interference pattern of light and shadow. Read on and become enlightened about diffraction.
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The Double-Slit experiment was first performed in 1801 by Thomas Young. Young passed sunlight through two narrow slits onto a screen. The light diffracted into semicircular waves as it passed through the narrow slits. Where the crests of these waves met the light on the screen was brighter, but where a crest met a trough a shadow appeared on the screen. This experiment helped support the wave theory of light. In modern physics, however, the double-slit experiment demonstrates that matter can demonstrate wave-like particles as well. Davisson and Germer demonstrated that electrons passed through slits exhibit diffraction in the same way light does.
Bragg’s Law results from a special case of diffraction. This form of diffraction, Bragg’s Diffraction, occurs when X-rays are scattered by the atoms of a crystalline solid. This scattering creates intense patterns of constructive interference. Bragg’s Law establishes a mathematical relationship between the wavelength of the X-ray, the crystal plane separation, and the diffraction angle at which diffraction is at its maximum.
FRAUNHOFER AND FRESNEL DIFFRACTION EQUATIONS
The Fraunhofer and Fresnel Diffraction equations model diffraction patterns in specific cases. The Fraunhofer Equation models a situation in which the aperture through which light passes is relatively far from the screen on which the diffraction pattern is observed. This is known as diffraction in the far field region. The Fresnel Diffraction Equation models diffraction in the near field region, where a diffraction pattern is viewed relatively close to the object or aperture that creates the diffraction. (You may recognize Fraunhofer from Fraunhofer Lines, which are absorption lines used to determine information about the sun by physicists.)
Diffraction patterns that form through a circular aperture, such as a camera or telescope, are characterized by a bright central region surrounded by rings of decreasing intensity. The bright central region in circular diffraction patterns is known as the Airy disc. The Airy Disc has a special relevance to astronomy, a profession in which circular apertures are extensively used to study distant light sources.
The Rayleigh Criterion is the minimum distance between two light sources for those light sources to be identified as distinct objects. If two light sources are viewed through a circular aperture and happen to be very close together, their Airy discs will overlap, and it will be impossible to differentiate them. The Rayleigh Criterion specifies that the central peaks of the light’s respective diffraction patterns must be farther apart than the radius of their Airy discs.
Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
Here’s some diffraction with an actual, real-life laser.
X-ray diffraction is responsible for incredible insight into the structure of molecules.
Light: it’s a wave! It’s a particle! It’s both!
For a concise demonstration of diffraction through a medium other than light, check out this video!
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